Press forming is a method of forming a blank to be press-formed by pressing a tool of press forming onto the blank to transfer the shape of the tool of press forming to the blank. The press forming gives, in some cases, after ejection of a press-formed part from the tool of press forming, rise to a drawback that the defect of shape caused by elastic recovery attributed to residual stress in the press-formed part, referred to as springback, occurs, and the shape of the press formed part varies from a desired shape.
The level of the springback to be generated is largely influenced by the strength of material in many cases. Recently, there has been an increasing tendency that the automotive industry particularly uses, in terms of the weight reduction of an automotive body, high-strength steel sheets for automotive body parts, and such high-strength steel sheets have increased the level of the springback to be generated. Accordingly, in order to bring the shape of the press-formed part after the springback occurs close to a design shape, the tool of press forming is required to be corrected many times by a skilled hand in a production site while repeating trial and error. As a result, the period of production is prolonged. Therefore, it is reasonable to say that the development of a method of effectively reducing the springback is a still more important task also in reducing the development period and cost of an automotive.
In reducing the springback, the control of the stress by which the springback is caused is indispensable. As a method that controls the stress to reduce the springback, for example, Patent Literature 1 discloses “PRESS-FORMING DIE APPARATUS OF STEEL SHEET.” The method described in Patent Literature 1 is a method of press-forming, in crash-forming a hat-shaped section part, the hat-shaped section part with a tool of press forming in which a projecting bead is formed in a flange portion. In this method, a blank is locked by the projecting bead immediately before a bottom dead center so as to impart tensile deformation to a side wall portion of the blank thus eliminating a stress difference in the thickness direction of the blank, the stress difference being a cause of the curl of the side wall portion.
Furthermore, as another example, there has been proposed a method of press-forming a blank with a tool of press forming that has a blank holder with a recess arranged on the periphery of a punch, in Patent Literature 2. In this method, a blank end is entered into the recess of the blank holder while press-forming and thereafter, the blank end abuts on an inner wall of the recess to be confined to the recess. Accordingly, the blank stops protruding out of the recess thus imparting in-plane compressive stress to a side wall portion of the blank immediately before a bottom dead center and eliminating a stress difference in the thickness direction of the blank.